1. Field of Invention
This invention is in the broad field of signal processing and, in particular, is in the field of interference reduction in a transmission line, or its equivalent, carrying an unwanted signal or interference.
2. Related Art
Frequency domain filters that discriminate against undesired signals constitute one related prior art for this invention, since when the unwanted signals occupy frequency bands different from that of the desired signal, the unwanted signals can filtered out of the transmission line. If however the desired signal and interferences occupy an overlapping frequency band, conventional filters discriminating on the basis of signal frequencies, are of little value. Also, when the power levels of the signal or the interference or both are very high, the design of a filter becomes a difficult task since even a slight mistuning of the tuned circuits in the filters may burn out such filters. The present invention relates to methods and apparatus that avoid the short comings of the frequency domain filters by providing an adaptive cancellation of the interference regardless of its spectral characteristics and power levels. Further distinction between the present invention and related prior art will be apparent from the specification, drawings, and claims of this invention.
The inventions of Chaplin.sup.1, Swinbanks.sup.2, Ghose and Sauter.sup.3 and Ghose.sup.4 should be considered as relevant references in connection with the present invention since they all address interference reduction. The difference between the present invention and each of these references are discussed below.
In many modern radars, the modulated, transmitting signal is often amplified to its full power level before it is fed to the antenna through the transmit-receive switch, or its equivalent for subsequent radiation. For such cases even a slight nonlinearity in the amplifier could generate intermodulation signal products having frequencies different from those of the desired transmitting signal, particularly for the high power level of the amplifier. These intermodulation products can be sources of interferences in neighboring communication and radar receivers, when such receivers are intended to receive very weak signals. When the radar waveform is complex, the intermodulation frequency spectrum extends far beyond that of the transmitting signal spectrum for the radar, and even when the power level of the interference is much below that of the transmitting signal, the interference effect is not tolerable in many circumstances.
When the interference and the desired transmitting signal spectra are not overlapping, a frequency domain allowing the desired signal to pass but restricting the interference, should theoretically remedy the interference reduction problem. However, such a filter has to be designed for the high power level of the transmitting signal. High power filters are expensive and often not reliable since even a slight mistuning of the filter circuit may burn such a filter. An alternative to this high power filter is a means which subtracts selectively the interference from the transmission line or waveguide feeding the antenna, leaving the desired transmitting signal at the output of the transmission line or waveguide and hence at the antenna. Since the power level of the interference is usually much lower than that of the transmitted signal, no high power filter will be needed in that case. One objective of the invention is to offer an adaptive alternative means to filter out an interference selectively from a high power transmission line or waveguide without using high power filter circuits. Such an alternative effects the interference reduction by selectively cancelling the interference by synthesizing an interference which is equal in amplitude but 180 degrees out-of-phase with respect to the interference to be cancelled. In this invention, the cancelling interference is synthesized from a sample of the interference at one point of the transmission line or waveguide, by reversing its polarity and adjusting its amplitude and time delay or phase until the adjusted interference becomes equal in amplitude and 180 degrees out-of-phase with respect to the interference remaining at the transmission line or waveguide at the coupling point where the cancelling interference is injected into the line or waveguide. The synthesis in the invention is effected by a closed-loop control discussed further in the text.
The concept of interference reduction from a transmission line or waveguide can be extended to the acoustic interference or noise reduction in an exhaust pipe of an auto or Diesel engine. At present noise mufflers are used at the exhaust pipe to reduce the intensity of acoustic interference generated in auto and Diesel engines. While reducing the acoustic noise, however, the muffler creates a back-pressure at the exhaust pipe which in turn leads to a reduction of engine efficiency, and more importantly, to an increased amount of unburned hydrocarbons. Many concerned people believe that an alternative means of noise reduction that can reduce unburned hydrocarbons in exhaust and increase engine efficiency, thus requiring even less fuel, may be highly desirable. Providing such an alternative is another objective of this invention. In this case the exhaust pipe is a transmission line for the acoustic interference or noise and the noise reduction principle involves, as noted earlier, sampling of the interference from a sampling point of the exhaust pipe, reversing its polarity and adjusting its amplitude and phase until the adjusted interference becomes equal in amplitude and 180 degrees out-of-phase of the same interference at a coupling point of the exhaust pipe, ahead of the sampling point where the adjusted interference is injected into the pipe. The sampling, in this case, is effected by a microphone which converts the acoustic interference into an electric signal while the injection of the adjusted interference is effected by a transducer which converts the electric signal back to the corresponding acoustic interference. Again, the synthesis of the cancelling signal is effected by a closed-loop control.
With reference to the prior art, the inventions of Chaplin.sup.1, Swinbanks.sup.2, Ghose and Sauter.sup.3, and Ghose.sup.4 should be considered as relevant since they all address the reduction of interference or noise by synthesizing a cancelling interference by various means. The similarity between any one of these inventions with the present invention, however, virtually ends there. For example, the Chaplin invention teaches us how to minimize repetitive vibration, presumably in a solid structure where the vibration is set up by a machine. Repetitive vibration characterizes back and forth motion in an elastic medium that is repeated at regular intervals of time. This implies, primarily, the presence of a single frequency as is assumed in the Chaplin invention when the "synchronizing" signal is obtained by a phased-lock loop. The phased-lock loop is capable of locking the phase of one signal with respect to another and, since the term "phase" is meaningless for a multifrequency incoherent noise or interference, the teaching of the Chaplin invention is confined to means of reducing primarily a single frequency vibration. Furthermore, the vibration in solids or in an elastic medium depends not only on the characteristics of the source that stimulates the vibration, but also on the material characteristics of the medium undergoing vibration, its shape and geometry such as solid bars, strings, membranes, solid structures, etc. Perhaps, the most fundamental difference between this invention and the Chaplin invention is that the former addresses the problem of reduction of simultaneously present multifrequency acoustics or electromagnetic interference or noise when the phases corresponding to these frequencies are incoherent and random, and change with time, while the Chaplin invention addresses a single frequency interference or noise.
Next, the Swinbank invention teaches us how to minimize acoustic interference or noise in a pipe, where unlike the Chaplin invention, the interference could contain multiple frequencies simultaneously. In Swinbank's invention, the transfer function of the signal processing system synthesizing the cancelling interference is varied in an iterative manner using a complex data processing and memory. Such a control to synthesize a cancelling interference is quasi closed-loop control at best, and the degree of cancellation becomes limited, particularly when a rapid change of transfer function is needed. In contrast, the equivalent transfer function in the present invention involves an amplitude gain or loss and a change in time delay or phase requiring no memory or complex data processing. Also, the control in this invention is truly a closed-loop control where the criterion of driving the error signal to zero is the same as the desired objective of interference reduction.
Similarly, unlike the present invention, the Ghose and Sauter invention relates to a radio communication system and is useful for minimizing or eliminating an interference in a radio receiver. Although there are some similarities between the present invention and that of Ghose and Sauter, the former invention addresses the problem of interference reduction where the desired signal is more powerful than the interference in the line to be protected while the Ghose and Sauter invention addresses the problem of interference reduction in a line where the interference is more powerful than the desired signal, particularly insofar as the electromagnetic interference is concerned. Also, unlike the present invention, the Ghose and Sauter invention cannot cure an acoustic interference problem, and unlike the Ghose and Sauter invention the desired signal is not necessary for the effective removal of the acoustic interference in the present invention.
Finally, the Ghose invention, unlike the present invention, addresses the problem of interference reduction over a broad region. It is more like a phased array where the amplitude and phase of different elements of the array are varied to create a null in a specific direction. Radiating sources of interference are vital elements of the Ghose invention. No such elements are involved in the present invention. The Ghose invention and the present invention are, therefore, two different inventions having different objectives, relevant elements and claims.
Other differences between the relevant prior arts and the present invention will be apparent from the specifications, drawings and claims that follow.
______________________________________ REFERENCES U.S. PATENT DOCUMENTS ______________________________________ (1) 4,566,118 01/1986 CHAPLIN 381/71 (2) 4,596,033 06/1989 SWINBANKS 381/71 (3) 3,699,444 10/1972 GHOSE AND SAUTER (4) 4,829,590 05/1989 GHOSE ______________________________________